1. Field of the Invention
This invention relates to polymeric insulation compositions which are crosslinked to produce heat resistant and flame resistant products useful for coating wire and cable products. More particularly, the invention relates to crosslinkable flame retardant ethylene-vinyl ester and ethylene-alkyl acrylate copolymer insulation compositions which do not tarnish the surface of copper conductors upon curing.
2. Description of the Prior Art
One of the most important uses of fire resistant polymer compositions is for wire and cable insulation. In electrical environments both insulating and fire resistant properties are considered to be necessary. Furthermore, the physical properties of the insulation should not deteriorate under the service conditions encountered. Various antioxidants/stabilizers are therefore added to the formulations so that they meet the service requirements.
A particularly important type of fire retarding insulation which is widely used by the wire and cable industry is comprised of a crosslinkable polymer, such as ethylene-vinyl acetate copolymer, one or more silanes, one or more hydrated inorganic fillers, and a crosslinking agent. Additives such as pigments, processing oils, lubricants, stabilizers and antioxidants are also generally included in the formulations. Such compositions are disclosed in U.S. Pat. Nos. 3,832,326 and 3,922,442 to North et al and U.S. Pat. Nos. 4,349,605 and 4,38i,362 to Biggs et al. The compositions of North et al and Biggs et al find particular use as insulation compositions which can be extruded onto copper conductors to provide a single layer insulating and jacketing composition which meets the automotive primary SAE J1128 standards and UL 125.degree. C. appliance wire and UL 44 SIS standards. The compositions exhibit a good balance of processability, improved physical and electrical properties together with a high degree of flame and fire retardance. Moreover, these results are achieved (a) without the use of halogenated flame retardant additives thereby eliminating the potential for generating dangerous hydrogen halide fumes; (b) without the use of carbon black thereby making it possible to formulate colored insulations; (c) without the application of flame retardant coatings thereby eliminating the need for an additional step in manufacturing operations after the insulating compound is extruded onto the conductor; and (d) without the use of antimony trioxide thereby eliminating the need to use a substantial quantity of an expensive compounding ingredient.
Antioxidants disclosed in the North et al and Biggs et al references include polymerized 1,2-dihydro-2,2,4-trimethyl quinoline, distearyl-3,3'-thiodiproponate (DSTDP), and combinations of DSTDP with hindered phenols, such as tetrakis [methylene(3,5-di-t-butyl-4-hydroxyhydrocinnamate)]methane. Use of DSTDP with sterically hindered phenols is disclosed in U.S. Pat. No. 4,381,362 for the preparation of compositions which pass the CSA varnish test. Other thio compounds, such as dilauryl-3,3'-thiodipropionate, dimyristylthiodipropionate, ditridecylthiodipropionate, bis alkyl sulfides, and hindered phenols, such as 2,6-di-t-butyl-p-cresol, octadecyl 3,5-di-t-butyl-4-hydroxyhydrocinnamate, 2,2'-methylene bis(6-t-butyl-4-methylphenol), 4,4'-butylidene bis(6-t-butyl-3-methyl phenol), 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl) benzene, and 2,2'-methylene bis(4-methyl-6-t-butylphenol) have also been disclosed.
While it is possible to obtain effectively stabilized white formulations using combinations of dialkylthiodipropionates and hindered phenols, discoloration of the copper conductor has been observed when the insulated wire or cable is cured using steam. While the exact nature of the discoloration or tarnish is not known, its presence on the surface of the conductor presents a problem when making connections by soldering or welding, such as in the manufacture of electrical boards or harnesses. The absence of a clear, shiny metal surface can result in defective connections and is a particular problem in automated systems. With sonic welding, for example, the presence of tarnish on the conductor surface makes it necessary to increase the power to achieve reliable welds. This, of course, results in higher cost and can lead to other problems.
Since continuous vulcanization (CV) procedures which utilize steam are widely used by wire and cable manufacturers, it would be desirable if a stabilizer package which did not tarnish the surface of the conductor were available for formulations containing organic peroxide crosslinking agents. Whereas certain combinations of thio compounds and hindered phenols do not discolor the copper conductor, they have other disadvantages. For example, bisalkyl sulfide/hindered phenol combinations do not tarnish the conductor but, at the level necessary to obtain effective stabilization, they have blooming and odor problems. It would be highly advantageous if an effective thio compound/hindered phenol stabilizer combination were available which could be formulated to produce essentially white compositions and which upon steam curing did not discolor the copper conductor or produce undesirable blooming or objectionable odor.